。
She played tennis all day yesterday.
Literal
She [topic-は] yesterday one-day-throughout tennis [object-を] did.
一日中 ('all day long') is built from 一日 ('one day') + 中 ('throughout, during'); the same 中 turns up in 一年中 (all year), 世界中 (all over the world), and 体中 (all over the body), with the meaning of complete coverage rather than a midpoint. テニス is an English loanword written in katakana per the standard convention; verbing it with をする makes a casual sentence — テニスをする, 野球をする, サッカーをする all follow this template.